Beamforming techniques are well known in radio communications systems in which a single receiving station must be capable of receiving signals from multiple signal sources, i.e., transmitters (or a single source, if that source is moving) for increasing the focus or gain of the receiver in the direction of the signal(s). In particular, a plurality of receiving elements, i.e., antennas, can be focused to receive signals in a particular angular portion of the receiving area while rejecting other signals and interference outside of that angular portion of the receiving area. This can be done by using multiple, directional antennas focused on different angular portions of the geographic area of reception surrounding the antenna array. Alternately, the electrical outputs of an array of omnidirectional receiving antennas can be electronically weighted and combined in various manners so as to create a plurality of beam signals, each beam being particularly focused on a particular angular portion of the geographic region surrounding the antenna array. The output signals of the antennas, of course, are radio frequency (RF) signals. These outputs may or may not be frequency downconverted to an intermediate frequency (IF) prior to any processing. Thus, the circuitry for combining the antenna output signals operates in the RF or IF band.
Beamforming is commonly used in wireless communication systems, such as in cellular telephone system base stations, for increasing reception performance. For instance, by focusing a receiving beam on a particular transmitter, that transmitter need transmit with much less power to be adequately received by the base station relative to a system using a base station with only a single omnidirectional antenna and no beamforming.
There are three general beamforming techniques in common use at this time, namely, fixed beamforming, adaptive beamforming and switched beamforming. Fixed beamforming can be implemented with an array of multiple, directional antennas or by use of an array of multiple, omnidirectional antennas with electronic beamforming circuitry. In fixed beam systems, the beams are permanent and fixed.
In adaptive beamforming systems, the beamforming is performed electronically, rather than physically with directional antennas. Further, the antenna electrical RF output signals are combined using adaptive combining circuitry such that a receiving beam can track a moving signal source.
Finally, in switched beam systems, the RF electrical output signals of multiple, omnidirectional antennas of an antenna array are combined by RF combining circuitry to generate a plurality of fixed beam output signals, each covering an angular portion of the geographic area surrounding the array. However, in switched beam beamforming techniques, an algorithm is employed to select the one of the fixed beam output signals that provides the best reception of the signal source(s) at any given instant in the region (the region being termed a cell in cellular communications systems). As conditions change, the selected beam also changes accordingly. The switching rates typically used in switched beam beamforming systems are typically substantially slower than the data rate of the system. One common scheme is to switch about once a frame or even slower, a frame typically comprising several hundreds of data symbols.
Switched beamforming systems are commonly in use today with base stations of cellular telephone communications systems, including code division multiple access (CDMA) cellular telephone communications systems.
FIG. 1 is a block diagram of a switched beam beamforming system in accordance with the prior art. An antenna array 12 comprises multiple antennas 121, 122, . . . , 12N that are arranged at a receiving station 10, e.g., at the top of a cellular telephone system cell tower. The outputs of those antennas are fed into an analog N×B beamforming circuit 14, wherein N is the number of antennas and B is the number of beams generated by the beamforming circuitry from the antenna signals. The analog beamforming circuitry 14 weighs and combines the RF antenna output signals on lines 131, 132, . . . , 13N in accordance with a scheme dictated by beamforming control circuit 121 to produce the beam signals 151, 152, . . . , 15B, each beam focused on an angular portion of the reception area. Beamforming circuit 121 may be a DSP executing a predetermined algorithm. The number of beams, B, typically is less than or equal to the number of antennas, N, in the antenna array. Each beam signal 151, 152, . . . , 15B is passed through frequency down converting circuitry 161, 162, . . . , 16B for converting the beam signals from the RF frequency range to the baseband range. Each of those signals is digitized by an analog to digital (AID) converter 181, 182, . . . , 18B. The outputs of the A/D converters are then each input to a fading multipath and multi-user channel estimation circuit, 201, 202, . . . , 20B. Each of those circuits generates L output signals for each of M simultaneous transmitters at use in the given geographic area, where L is the number of paths per transmitter that the circuitry is designed to process simultaneously (e.g., typically around 3 or 4) and M is the number of simultaneous transmitters using the receive station. Those outputs are input to a minimum variance selector 22 for all of the paths and users. The minimum variance selector generates a path estimate and a path estimate error for each path of each user, i.e., L×M path estimates and L×M path estimate errors.
It should be understood by those skilled in the related arts that all circuitry subsequent to the analog-to-digital converters, i.e., circuits 201, 202, . . . , 20B and 22 typically would be digital and may be implemented by a digital signal processor (DSP), a processor, a microprocessor, an application specific integrated circuit, a finite state machine, a programmed general purpose computer or any other equivalent (hereinafter collectively “DSP”). In fact, because of the large amount of processing needed to generate the path estimates and path estimate errors from the beams, the flat fading multipath and multiuser estimation and the minimum variance selection typically is performed by a bank of DSPs.
In the analog N×B beamforming circuitry 14, for each beam, dedicated RF combining circuitry is necessary, i.e., there are B copies of essentially identical circuitry for processing the N beams on lines 151, 152, . . . , 15B. RF band hardware is much more expensive than DSPs and other baseband circuitry. Also, as shown in FIG. 1, each beam requires a dedicated frequency down converting circuit 16 and a dedicated analog-to-digital converter 18.
Accordingly, it is an object of the present invention to provide an improved switched beam beamforming method and apparatus.
It is another object of the present invention to provide a switched beam beamforming apparatus which reduces the amount of RF or IF band circuitry needed.
It is a further object of the present invention to provide a lower cost switched beam beamforming method and apparatus.
It is yet another object of the present invention to provide a switched beamforming method and apparatus in which the bulk of the signal processing requirements are transferred from the RF or IF band to the baseband.
It is yet a further object of the present invention to provide a switched beam beamforming method and apparatus having equivalent or superior performance to prior art techniques utilizing much lower cost circuitry components.